This invention relates to chemical reactors for the conversion of a reaction fluid while replacing catalyst and indirectly exchanging heat with a heat exchange fluid.
In many industries, like the petrochemical and chemical industries for instance, the processes employ reactors in which chemical reactions are effected in the components of one or more reaction fluids in contact with a catalyst under given temperature and pressure conditions. Most of these reactions generate or absorb heat to various extents and are, therefore, exothermic or endothermic. The heating or chilling effects associated with exothermic or endothermic reactions can positively or negatively affect the operation of the reaction zone. The negative effects can include among other things: poor product production, deactivation of the catalyst, production of unwanted by-products and, in extreme cases, damage to the reaction vessel and associated piping. More typically, the undesired effects associated with temperature changes will reduce the selectivity or yield of products from the reaction zone.
One solution to the problem has been the indirect heating of reactants and/or catalysts within a reaction zone with a heating or cooling medium. The most well known catalytic reactors of this type are tubular arrangements that have fixed or moving catalyst beds. The geometry of tubular reactors poses layout constraints that require large reactors or limit throughput.
Indirect heat exchange has also been accomplished using thin plates to define alternate channels that retain catalyst and reactants in one set of channels and a heat transfer fluid in alternate channels for indirectly heating or cooling the reactants and catalysts. Heat exchange plates in these indirect heat exchange reactors can be flat or curved and may have surface variations such as corrugations to increase heat transfer between the heat transfer fluids and the reactants and catalysts. Although the thin heat transfer plates can, to some extent, compensate for the changes in temperature induced by the heat of reaction, not all indirect heat transfer arrangements are able to offer the complete temperature control that would benefit many processes by maintaining a desired temperature profile through a reaction zone. Many hydrocarbon conversion processes will operate more advantageously by maintaining a temperature profile that differs from that created by the heat of reaction. In many reactions, the most beneficial temperature profile will be obtained by substantially isothermal conditions. In some cases, a temperature profile directionally opposite to the temperature changes associated with the heat of reaction will provide the most beneficial conditions. An example of such a case is in dehydrogenation reactions wherein the selectivity and conversion of the endothermic process is improved by having a rising temperature profile or reverse temperature gradient through the reaction zone. A specific arrangement for heat transfer and reactant channels that offers more complete control can be found in U.S. Pat. No. 5,525,311; the contents of which are hereby incorporated by reference.
Most catalysts for the reaction of hydrocarbons are susceptible to deactivation over time. Deactivation will typically occur because of an accumulation of deposits that cause deactivation by blocking active pore sites or catalytic sites on the catalyst surface. Where the accumulation of coke deposits causes the deactivation, reconditioning the catalyst to remove coke deposits restores the activity of the catalyst. Coke is normally removed from the catalyst by contact of the coke-containing catalyst at high temperature with an oxygen-containing gas to combust or remove the coke in a regeneration process. The regeneration process can be carried out in situ or the catalyst may be removed from a vessel in which the hydrocarbon conversion takes place and transported to a separate regeneration zone for coke removal. Arrangements for continuously or semi-continuously removing catalyst particles from a bed in a reaction zone for coke removal in a regeneration zone are well known. U.S. Pat. No. 3,652,231 describes a continuous catalyst regeneration process which is used in conjunction with catalytic reforming of hydrocarbons; the teachings of which are hereby incorporated by reference. In the case of the reaction zone, the catalyst is transferred under gravity flow by removing catalyst from the bottom of the reaction zone and adding catalyst to the top.
A phenomenon known as xe2x80x9cpinningxe2x80x9d inhibits catalyst transfer in many reactor arrangements. xe2x80x9cPinningxe2x80x9d is the phenomenon wherein the flow of reactant gas at sufficient velocity can block the downward movement of catalyst. xe2x80x9cPinningxe2x80x9d is a function of the gas velocity and the physical characteristics of the flow channel in which the flowing gaseous reactants contact the catalyst. As the gas flows through the channels that retain the catalyst, the gas impacts the catalyst particles and raises intergranular friction between the particles. When the vertical component of the frictional forces between the particles overcomes the force of gravity on the particles the particles become pinned. As the flow path length of gas through the catalyst particles becomes longer, the forces on the particles progressively increase from the outlet to the inlet of the flow channel. In addition, as the catalyst flow channel becomes more confined, the gravity flow of catalyst particles becomes more hindered. Accordingly, as the size of the flow channel becomes more confined, wall effects increasingly add to the vertical hold-up force on the catalyst particles. As a result narrow flow channels have a greater susceptibility to pinning and cannot normally provide continuous catalyst circulation.
In the case of reactors providing indirect heat exchange, the arrangement of the reactor exacerbates the problem of catalyst pinning. Increasing the number of channels by decreasing their size facilitates heat transfer by increasing the surface area between the heat exchange fluid and the catalyst. In addition, heat transfer is further facilitated by irregularities in the plate surface that create turbulence and reduce film factors that interfere with heat exchange. However, irregularities in the plates that define the channels further interfere with the movement of catalyst and promote a greater tendency for the catalyst to xe2x80x9cpin.xe2x80x9d Therefore, methods and reactor arrangements are sought to use a channel type reactor that facilitates heat exchange and catalyst circulation while the reactor continues operation.
Accordingly, it is an object of this invention to provide a process for the contact of reactants with a bed of catalyst while providing indirect heat exchange with a heat exchange fluid and on stream circulation of the catalyst.
It is a further object of this invention to provide a reactor apparatus for the indirect heat exchange of a reactant stream and contact of the reactant stream with a bed of catalyst while allowing on stream circulation of the catalyst.
This invention uses a sequential gas flow reduction or cessation, to periodically remove catalyst from selected reactant channels in a single reaction zone so that continuous circulation can be effected in the reaction zone while maintaining passage of reactants through the catalyst and while indirectly exchanging heat between reactants and a heat exchange medium. Subdivision of the reaction zone into a multiplicity of reaction stacks provides multiple reactant channel banks for selective flow reduction or cessation during catalyst transfer. The reaction stacks define alternating channel passageways. The channel passageways extend vertically and horizontally. Catalyst enters the reactant passageways and is continuously or semi-continuously removed from the bottom of the passageway to effect catalyst circulation. The reactants flow radially through the reactant passageways for contact with the catalyst. Plates defining the passageways provide a heat transfer surface for a heat transfer fluid that passes through the heat transfer channels. The withdrawal and/or addition of reactants to the reaction stacks is selectively controlled so that the flow of reactants to one or more reaction stacks is interrupted or restricted while the flow of reactants continues in the remaining reaction stacks. Restriction or interruption of the flow permits the catalyst to drop under gravity flow from the selected reactions zones.
The sequencing of reactant flow restriction and the withdrawal of catalyst may be accomplished in any manner that suits the particular process. The cycling of reactant flow reduction and the removal of catalyst particles may proceed continuously with sequential catalyst withdrawal from each reaction stack on a regular interval. Alternately, the process operation may continue until a predetermined degree of deactivation results. At such time, a sequencing of reactant flow restriction may be used to establish a cycle that replaces catalyst sequentially in each reaction zone until a desired degree of activity is again reestablished.
A combination of catalyst replacement and indirect heat exchange with a heat transfer fluid can also provide a reaction advantage for processes that use this invention. This combination can provide an isokinetic reaction condition within the reaction stacks. As catalyst is incrementally replaced in the reaction stacks, the most deactivated catalyst is removed from the bottom while the most active catalyst enters the top of the reaction stack. This periodic replacement thereby provides a continuous activity gradient down the length of the catalyst bed in each reaction stack. The decrease in activity can be compensated for by an increase in the reaction temperature. In the case of an endothermic reaction where a heating fluid enters the heat transfer channels, the fluid can enter the reaction stack in a flow direction that compensates for the loss of activity in the catalyst. By passing the heat exchange fluid from the bottom of the reaction stack to the top of the reaction stack, higher temperatures are maintained in the lower portion where the more deactivated catalyst contacts reactants. Progressing upwardly through the reaction stack, heating of the reactants cools the heating medium thereby resulting in a relatively reduced temperature for the reactants in the upper portion of the reaction stack which contains the most active catalyst. Tailoring of catalyst replacement, heating medium temperature, and heat exchange across the reactant and heat exchange channels can be arranged to provide an isokinetic operation across the reaction zone. This isokinetic operation can result in a more uniform product effluent and the most efficient utilization of the reaction volume in each reaction stack. Isokinetic conditions can be maintained with exothermic reactions as well as with endothermic reactions. In exothermic reactions, the cooling medium should enter the top of the heat exchange channels to maintain a co-current flow with the catalyst so that the maximum cooling is provided at the region of the most active catalyst.
Accordingly, in one embodiment, this invention is a process for contacting reactants with catalyst in a channel reactor and indirectly contacting the reactants with a heat transfer fluid that permits intermittent movement of catalyst through a catalyst bed. In the process catalyst particles are retained in a plurality of reactions stacks. Each reaction stack has a plurality of vertically and horizontally extended reaction channels and heat exchange channels. A reactant stream passes to at least one of the reactant stacks and contacts the catalyst with the reactant stream. A product stream is recovered from the reactant channels. A heat exchange fluid passes through the heat exchange channels, and an at least partial restriction of the flow of the reactant stream to a selected reaction stack is intermittently effected to transfer catalyst particles in the selected reaction stack by withdrawing catalyst particles from the bottom and adding catalyst particles to the top of the reaction stack. The flow of the reactant stream is reestablished to the reactant stack after the addition of catalyst thereto.
In another embodiment, this invention is a channel reactor arrangement for contacting reactants with a particulate catalyst, indirectly heat exchanging the reactants with a heat transfer fluid, and replacing catalyst particles on stream. The arrangement includes a plurality of reaction stacks made up of parallel plates that extend vertically and horizontally to define heat transfer channels and reactant channels in each reactant stack. Means are provided for passing a reactant stream through the reactant channels and selectively restricting the flow of the reactant stream through selected reactant channels. Means are also provided for passing and adding catalyst particles to the top and withdrawing catalyst particles from the bottom of each reaction stack. The reactant channels also work in cooperation with means for passing a heat exchange fluid through the heat exchange channels of each reaction stack.
In another embodiment, this invention is a reactor for contacting reactants with a particulate catalyst while indirectly heat exchanging the reactants with a heat exchange fluid and replacing catalyst particles on stream. The apparatus contains a reactor vessel that houses a plurality of reaction stacks. Each reaction stack comprises a plurality of parallel plates that extend vertically and horizontally to define heat transfer channels and reactant channels in each reaction stack. The reactor vessel defines a reactant inlet for passing a reactant stream into the reactant channels. At least two manifolds receive fluid from the reactant channels. Each manifold is in communication with a valve for regulating the flow of fluid from the reactant channels and each manifold communicates with less than the total number of reactant channels. A catalyst distributor at the top of each reactant stack and a catalyst collector at the bottom of each reactant stack operates in conjunction with means for selectively controlling the addition and withdrawal of catalyst particles to and from each reactant stack. Means are also provided for passing a heat exchange fluid through the heat exchange channels in each reaction stack.
Additional embodiments, arrangements, and details of this invention are disclosed in the following detailed description of the invention.